1. Field of the Invention
The present invention relates to polyurethane adhesives which are useful in bonding porous and nonporous surfaces and to a method of bonding two or more substrates together with a polyurethane adhesive. More particularly, the present invention relates to polyurethane adhesives which are especially useful in bonding windshield glass into an automotive frame under varied environmental conditions and to a method of bonding windshield glass to an automotive frame using a polyurethane adhesive.
2. Brief Description of Art
A variety of adhesives, including polyurethanes, are useful for bonding to porous and non-porous substrates (see, e.g., U.S. Pat. Nos. 4,374,237, 4,687,533, 5,603,798, 5,672,652 and 5,852,103, each of which is incorporated herein by reference). Polyurethane adhesive compositions typically comprise at least one polyurethane prepolymer. Both one and two-part polyurethanes are known and may be used to bond a variety of substrates, including glass to metal.
In general, two-component urethane adhesive systems can provide curing which is less dependent upon weather condition than one-component urethane adhesive systems. For this reason, a two-component system is generally thought to have more flexibility under a variety of application conditions, especially during the winter as compared with a one-part urethane adhesive.
Two-part urethane adhesives, however, typically require special application equipment. For example, a special application gun with an electrically powered mixing mechanism which mixes the two parts in a specific ratio prior to application may be required. Such applicators are more difficult and costly to use than applicators for one-part urethane adhesives. Further, the improper mixing of two-component urethane system can result in poor final physical properties of the adhesives and even in poor adhesion. The use of a two-part system at high temperature may also be disadvantageous since a too rapid initial cure may provide such a urethane adhesive with a short working time.
In comparison, a one-part urethane adhesive provides improved ease of use since moisture cures the adhesive without any added mechanical mixing. Conventional one-part polyurethane adhesives are strongly dependent on the environmental conditions because of the need for such moisture curing. Hardening and in turn the amount of time needed before an adhesive product may be used are therefore affected by weather conditions. This is particularly true in the winter time where low temperature and low humidity conditions reduce the curing rate of one-component polyurethane adhesives no matter what kind of catalyst and how much of it is used in the formulation. In addition, using too much strong catalyst usually adversely impacts storage stability and can also dramatically shorten the working time at high temperature and high humidity conditions.
The use of thermoplastic materials in polyurethanes for one-part adhesives further allows easy application at elevated temperatures and improved green strength once the material cools down. One example of the use of thermoplastic materials in polyurethane-based adhesives is Hot-Melt Thermoplastic Urethane Elastomers (Hot Melt TPU). See, e.g., U.S. Pat. No. 5,936,018. Thermoplastic urethanes (TPU) are reacted and neutral materials, which need high temperatures (usually >100° C.) to become soft/viscous and have very fast setting times (usually a few seconds to several minutes). Another example of a reactive hot-melt material is an isocyanate group end-capped urethane prepolymer comprising thermoplastic polyester-polyol (e.g., see EP 0-909-771 A1).
In the automotive after-market replacement industry, glass may be bound into automobiles through the use of either one-part or two-component curable polyurethane adhesives. Two-part polyurethane adhesives may be used since they offer rapid initial cure allowing for rapid drive-away times. Two-part polyurethane adhesives are well known for this application; see, e.g., U.S. Pat. Nos. 4,835,012, 5,672,652 and 5,852,137, incorporated herein by reference.
Despite the relatively rapid drive-away times afforded by two-part polyurethane adhesives, faster curing polyurethane adhesives which allow even faster drive-away times are in demand. For instance, safe drive-away times of about 60 minutes or less from application, and more preferably about 30 minutes or less from application, are preferred. However, a trade-off exists between cure time, adhesive strength and working time, generally defined as the period from application of an adhesive until the adhesive becomes too intractable to work with. For example, if an adhesive cures too rapidly, a window installer may lose the necessary time to install and properly place the glass into the frame before the adhesive becomes too intractable to work with. For these reasons, the minimum working time for such windshield replacements is about 6 to 12 minutes.
A one-part moisture curable polyurethane is preferably used when glass is installed in automobiles since the equipment needed for the application of such an adhesive in an either automobile assembly plant or as an after market replacement is less expensive than the equipment needed to apply a two-component adhesive. One part polyurethane adhesives are disclosed in U.S. Pat. Nos. 4,374,237, 4,687,533, 5,922,809, 6,133,395 and 6,133,398, the disclosures of which are herein incorporated by reference.
Generally, thermoplastic urethanes, either TPU's or prepolymers, will boost both green strength, which is desired, and the compression force required to install such a windshield, which is not desired. As mentioned above, higher green strength may shorten safe drive-away times, but higher compression forces required to install the windshield may shorten the working time.
Federal Transportation Agency regulations require windshields to remain in place at crashes of up to 30 mph (48 km/h). Federal Motor Vehicle Safety Standards (FMVSS) 208, 212 and 216 describe certain requirements which are pertinent to automotive windshield replacements. In particular, these standards set forth the minimum performance characteristics required of a windshield retention system. To meet such standards, it has been shown through crash studies of the forces on a windshield, both due to deceleration and airbag impact on the windshield, that a tensile strength of the adhesive should be preferably at least about 145 psi (1.0 MPa) at the designated drive away time measured at a strain rate of 1 meter/second.
For at least the above reasons, it is desirable to provide a one-part polyurethane adhesive which facilitates faster safe drive-away times, which meets the strength requirements to allow for the safe replacement of automotive windshields and which still provides for a reasonable working time to facilitate proper placement of glass in window frames.